Mixing and Heat Integration of Melt Tray Liquids in a Cryogenic Distillation Tower

ABSTRACT

A cryogenic distillation tower for separating a feed stream. The tower includes a distillation section. A controlled freeze zone section is situated above the distillation section and forms a solid from the feed stream. The controlled freeze zone section includes a spray assembly in an upper section and a melt tray assembly in a lower section. The melt tray assembly includes at least one vapor stream riser that directs the vapor from the distillation section into liquid retained by the melt tray assembly, and one or more draw-off openings positioned to permit a portion of the liquid to exit the controlled freeze zone section. The portion of the liquid indirectly exchanges heat with a heating fluid. One or more return inlets return the portion of the liquid to the melt tray assembly after it has been heated in the heat exchanger.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of United StatesProvisional Patent Application No. 62/691,679 filed Jun. 29, 2018,entitled MIXING AND HEAT INTEGRATION OF MELT TRAY LIQUIDS IN A CRYOGENICDISTILLATION TOWER.

This application is related to but does not claim priority to U.S.Provisional patent application numbers: 61/912,975 Filed Dec. 6, 2013and titled METHOD AND SYSTEM FOR SEPARATING A FEED STREAM WITH A FEEDSTREAM DISTRIBUTION MECHANISM; 61/912,957 filed on Dec. 6, 2013 andtitled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTSWITH A SPRAY ASSEMBLY; 62/044,770 filed on Sep. 2, 2014 and titledMETHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH ASPRAY ASSEMBLY; 61/912,959 filed on Dec. 6, 2013 and titled METHOD ANDSYSTEM OF MAINTAINING A LIQUID LEVEL IN A DISTILLATION TOWER; 61/912,964filed on Dec. 6, 2013 and titled METHOD AND DEVICE FOR SEPARATING A FEEDSTREAM USING RADIATION DETECTORS; 61/912,970 filed on Dec. 6, 2013 andtitled METHOD AND SYSTEM OF DEHYDRATING A FEED STREAM PROCESSED IN ADISTILLATION TOWER; 61/912,978 filed on Dec. 6, 2013 and titled METHODAND SYSTEM FOR PREVENTING ACCUMULATION OF SOLIDS IN A DISTILLATIONTOWER; 61/912,983 filed on Dec. 6, 2013 and titled METHOD OF REMOVINGSOLIDS BY MODIFYING A LIQUID LEVEL IN A DISTILLATION TOWER; 61/912,984filed on Dec. 6, 2013 and titled METHOD AND SYSTEM OF MODIFYING A LIQUIDLEVEL DURING START-UP OPERATIONS; 61/912,986 filed on Dec. 6, 2013 andtitled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTSWITH A HEATING MECHANISM TO DESTABILIZE AND/OR PREVENT ADHESION OFSOLIDS; and 61/912,987 filed on Dec. 6, 2013 and titled METHOD ANDDEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH A SURFACETREATMENT MECHANISM.

This application is also related to U.S. Application No. 62/691,676,titled HYBRID TRAY FOR INTRODUCING A LOW CO2 FEED STREAM INTO ADISTILLATION TOWER, having common inventors with this application, andfiled on the same date as this application.

BACKGROUND Fields of Disclosure

The disclosure relates generally to the field of fluid separation. Morespecifically, the disclosure relates to the cryogenic separation ofcontaminants, such as acid gas, from a hydrocarbon.

Description of Related Art

This section is intended to introduce various aspects of the art, whichmay be associated with the present disclosure. This discussion isintended to provide a framework to facilitate a better understanding ofparticular aspects of the present disclosure. Accordingly, it should beunderstood that this section should be read in this light, and notnecessarily as admissions of prior art.

The production of natural gas hydrocarbons, such as methane and ethane,from a reservoir oftentimes carries with it the incidental production ofnon-hydrocarbon gases. Such gases include contaminants, such as at leastone of carbon dioxide (“CO₂”), hydrogen sulfide (“H₂S”), carbonylsulfide, carbon disulfide and various mercaptans. When a feed streambeing produced from a reservoir includes these contaminants mixed withhydrocarbons, the stream is oftentimes referred to as “sour gas.”

Many natural gas reservoirs have relatively low percentages ofhydrocarbons and relatively high percentages of contaminants.Contaminants may act as a diluent and lower the heat content ofhydrocarbons. Some contaminants, like sulfur-bearing compounds, arenoxious and may even be lethal. Additionally, in the presence of watersome contaminants can become quite corrosive.

It is desirable to remove contaminants from a stream containinghydrocarbons to produce sweet and concentrated hydrocarbons.Specifications for pipeline quality natural gas typically call for amaximum of 2-4% CO₂ and ¼ grain H₂S per 100 scf (4 ppmv) or 5 mg/Nm3H₂S. Specifications for lower temperature processes such as natural gasliquefaction plants or nitrogen rejection units typically require lessthan 50 ppm CO₂.

The separation of contaminants from hydrocarbons is difficult andconsequently significant work has been applied to the development ofhydrocarbon/contaminant separation methods. These methods can be placedinto three general classes: absorption by solvents (physical, chemicaland hybrids), adsorption by solids, and distillation.

Separation by distillation of some mixtures can be relatively simpleand, as such, is widely used in the natural gas industry. However,distillation of mixtures of natural gas hydrocarbons, primarily methane,and one of the most common contaminants in natural gas, carbon dioxide,can present significant difficulties. Conventional distillationprinciples and conventional distillation equipment are predicated on thepresence of only vapor and liquid phases throughout the distillationtower. The separation of CO₂ from methane by distillation involvestemperature and pressure conditions that result in solidification of CO₂if a pipeline or better quality hydrocarbon product is desired. Therequired temperatures are cold temperatures typically referred to ascryogenic temperatures.

Certain cryogenic distillations can overcome the above mentioneddifficulties. These cryogenic distillations provide the appropriatemechanism to handle the formation and subsequent melting of solidsduring the separation of solid-forming contaminants from hydrocarbons.The formation of solid contaminants in equilibrium with vapor-liquidmixtures of hydrocarbons and contaminants at particular conditions oftemperature and pressure takes place in a controlled freeze zone (CFZ)section. A lower section may also help separate the contaminants fromthe hydrocarbons but the lower section is operated at a temperature andpressure that does not form solids.

In known cryogenic distillation applications using a CFZ section, a feedstream is dried and precooled to about −60° F. before introduction tothe distillation tower below the CFZ section and melt tray. The vaporcomponent of the cooled feed stream combines with the vapor rising fromthe stripping section of the tower and bubbles through the liquid on themelt tray. This serves several beneficial purposes, including: therising vapor stream is cooled and a portion of the CO₂ is condensed,resulting in a cooler and cleaner gas stream entering the open portionof the CFZ spray chamber; the rising vapor stream is evenly distributedacross the tower cross section as it enters the CFZ spray chamber; mostof the required melt tray heat input is provided via sensible heat fromcooling the vapor and latent heat from condensing a portion of the CO₂in the gas stream; and the melt tray liquid is vigorously mixed, whichfacilitates melting of solid CO₂ particles falling into the melt traywith the bulk liquid temperature only 2 to 3 degrees F. above themelting point of CO₂.

To achieve reliable melting of solids falling into the melt tray liquid,it is important that the melt tray liquid be maintained slightly abovethe melting point of CO₂ and well mixed. In a standard cryogenicdistillation design having a CFZ section, the mixing is achieved viavapor bubbling through the liquid. This vapor also provides the majorityof the heat input required for the melt tray. A heating coil immersed inthe melt tray liquid is used for fine tune control.

SUMMARY

The present disclosure provides a device and method for separatingcontaminants from hydrocarbons, among other things.

In an aspect, a cryogenic distillation tower is provided for separatinga feed stream. A distillation section permits vapor to rise upwardlytherefrom. One or more lines direct the feed stream into thedistillation tower. A controlled freeze zone section is situated abovethe distillation section. The controlled freeze zone is constructed andarranged to form a solid from the feed stream. The controlled freezezone section includes a spray assembly in an upper section of thecontrolled freeze zone, and a melt tray assembly in a lower section ofthe controlled freeze zone. The melt tray assembly includes at least onevapor stream riser that directs the vapor from the distillation sectioninto liquid retained by the melt tray assembly, and one or more draw-offopenings positioned to permit a portion of the liquid retained by themelt tray assembly to exit the controlled freeze zone section. A heatexchanger heats the portion of the liquid through indirect heat exchangewith a heating fluid. One or more return inlets return the portion ofthe liquid to the melt tray assembly after the portion of the liquid hasbeen heated in the heat exchanger.

In another aspect, a method is provided for cryogenically separatingcontaminants from a feed stream in a distillation tower. The feed streamis directed into the distillation tower. Vapor is permitted to riseupwardly from a distillation section of the distillation tower. A solidis formed in a controlled freeze zone section of the distillation tower.The controlled freeze zone section is situated above the distillationsection. The solid comprises contaminants in the feed stream. The vaporfrom the distillation section is directed into liquid retained by a melttray assembly using at least one vapor stream riser. The solid is meltedusing the liquid retained by the melt tray assembly. A portion of theliquid retained by the melt tray assembly is permitted to exit thecontrolled freeze zone section. The portion of the liquid is heatedthrough indirect heat exchange with a heating fluid in a heat exchanger.The portion of the liquid is returned to the melt tray assembly afterthe liquid has been heated in the heat exchanger.

The foregoing has broadly outlined the features of the presentdisclosure in order that the detailed description that follows may bebetter understood. Additional features will also be described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the disclosure willbecome apparent from the following description, appending claims and theaccompanying drawings, which are briefly described below.

FIG. 1 is a schematic diagram of a tower with sections within a singlevessel.

FIG. 2 is a schematic diagram of a tower with sections within multiplevessels.

FIG. 3 is a schematic diagram of a tower with sections within a singlevessel.

FIG. 4 is a schematic diagram of a tower with sections within multiplevessels.

FIG. 5 is a side view of a middle controlled freeze zone section of adistillation tower.

FIG. 6 is a top plan view of a melt tray according to disclosed aspects.

FIG. 7 is a flowchart of a method according to disclosed aspects.

It should be noted that the figures are merely examples and nolimitations on the scope of the present disclosure are intended thereby.Further, the figures are generally not drawn to scale, but are draftedfor purposes of convenience and clarity in illustrating various aspectsof the disclosure.

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of thedisclosure, reference will now be made to the features illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thedisclosure is thereby intended. Any alterations and furthermodifications, and any further applications of the principles of thedisclosure as described herein are contemplated as would normally occurto one skilled in the art to which the disclosure relates. It will beapparent to those skilled in the relevant art that some features thatare not relevant to the present disclosure may not be shown in thedrawings for the sake of clarity.

As referenced in this application, the terms “stream,” “gas stream,”“vapor stream,” and “liquid stream” refer to different stages of a feedstream as the feed stream is processed in a distillation tower thatseparates methane, the primary hydrocarbon in natural gas, fromcontaminants. Although the phrases “gas stream,” “vapor stream,” and“liquid stream,” refer to situations where a gas, vapor, and liquid ismainly present in the stream, respectively, there may be other phasesalso present within the stream. For example, a gas may also be presentin a “liquid stream.” In some instances, the terms “gas stream” and“vapor stream” may be used interchangeably.

As utilized herein, the terms “approximately,” “about,” “substantially,”and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numeral ranges provided. Accordingly, these terms should beinterpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described areconsidered to be within the scope of the disclosure.

The articles “the”, “a” and “an” are not necessarily limited to meanonly one, but rather are inclusive and open ended so as to include,optionally, multiple such elements.

The disclosure relates to a system and method for separating a feedstream in a distillation tower. The system and method helps optimallymatch where the feed stream should enter the distillation tower based onthe concentrations of components in the feed stream so as to improveenergy efficiency and/or optimally size the distillation tower. Thesystem and method may also help prevent the undesired accumulation ofsolids in the controlled freeze zone section of the distillation tower.FIGS. 1-7 of the disclosure display various aspects of the system andmethod.

The system and method may separate a feed stream having methane andcontaminants. The system may comprise a distillation tower 104, 204(FIGS. 1-2). The distillation tower 104, 204 may separate thecontaminants from the methane.

The distillation tower 104, 204 may be separated into three functionalsections: a lower section 106, a middle controlled freeze zone section108 and an upper section 110. The distillation tower 104, 204 mayincorporate three functional sections when the upper section 110 isneeded and/or desired.

The distillation tower 104, 204 may incorporate only two functionalsections when the upper section 110 is not needed and/or desired. Whenthe distillation tower does not include an upper section 110, a portionof vapor leaving the middle controlled freeze zone section 108 may becondensed in a condenser 122 and returned as a liquid stream via a sprayassembly 129. Moreover, lines 18 and 20 may be eliminated, elements 124and 126 may be one and the same, and elements 150 and 128 may be one andthe same. The stream in line 14, now taking the vapors leaving themiddle controlled freeze section 108, directs these vapors to thecondenser 122.

The lower section 106 may also be referred to as a stripper section. Themiddle controlled freeze zone section 108 may also be referred to as acontrolled freeze zone section. The upper section 110 may also bereferred to as a rectifier section.

The sections of the distillation tower 104 may be housed within a singlevessel (FIGS. 1 and 3). For example, the lower section 106, the middlecontrolled freeze zone section 108, and the upper section 110 may behoused within a single vessel 164.

The sections of the distillation tower 204 may be housed within aplurality of vessels to form a split-tower configuration (FIGS. 2 and4). Each of the vessels may be separate from the other vessels. Pipingand/or another suitable mechanism may connect one vessel to anothervessel. In this instance, the lower section 106, middle controlledfreeze zone section 108 and upper section 110 may be housed within twoor more vessels. For example, as shown in FIGS. 2 and 4, the uppersection 110 may be housed within a single vessel 254 and the lower andmiddle controlled freeze zone sections 106, 108 may be housed within asingle vessel 264. When this is the case, a liquid stream exiting theupper section 110, may exit through a liquid outlet bottom 260. Theliquid outlet bottom 260 is at the bottom of the upper section 110.Although not shown, each of the sections may be housed within its ownseparate vessel, or one or more section may be housed within separatevessels, or the upper and middle controlled freeze zone sections may behoused within a single vessel and the lower section may be housed withina single vessel, etc. When sections of the distillation tower are housedwithin vessels, the vessels may be side-by-side along a horizontal lineand/or above each other along a vertical line.

The split-tower configuration may be beneficial in situations where theheight of the distillation tower, motion considerations, and/ortransportation issues, such as for remote locations, need to beconsidered. This split-tower configuration allows for the independentoperation of one or more sections. For example, when the upper sectionis housed within a single vessel and the lower and middle controlledfreeze zone sections are housed within a single vessel, independentgeneration of reflux liquids using a substantially contaminant-free,largely hydrocarbon stream from a packed gas pipeline or an adjacenthydrocarbon line, may occur in the upper section. And the reflux may beused to cool the upper section, establish an appropriate temperatureprofile in the upper section, and/or build up liquid inventory at thebottom of the upper section to serve as an initial source of sprayliquids for the middle controlled freeze zone section. Moreover, themiddle controlled freeze zone and lower sections may be independentlyprepared by chilling the feed stream, feeding it to the optimal locationbe that in the lower section or in the middle controlled freeze zonesection, generating liquids for the lower and the middle controlledfreeze zone sections, and disposing the vapors off the middle controlledfreeze zone section while they are off specification with too high acontaminant content. Also, liquid from the upper section may beintermittently or continuously sprayed, building up liquid level in thebottom of the middle controlled freeze zone section and bringing thecontaminant content in the middle controlled freeze zone section downand near steady state level so that the two vessels may be connected tosend the vapor stream from the middle controlled freeze zone section tothe upper section, continuously spraying liquid from the bottom of theupper section into the middle controlled freeze zone section andstabilizing operations into steady state conditions. The split towerconfiguration may utilize a sump of the upper section as a liquidreceiver for the pump 128, therefore obviating the need for a liquidreceiver 126 in FIGS. 1 and 3.

The system may also include a heat exchanger 100 (FIGS. 1-4). The feedstream 10 may enter the heat exchanger 100 before entering thedistillation tower 104, 204. The feed stream 10 may be cooled within theheat exchanger 100. The heat exchanger 100 helps drop the temperature ofthe feed stream 10 to a level suitable for introduction into thedistillation tower 104, 204.

The system may include an expander device 102 (FIGS. 1-4). The feedstream 10 may enter the expander device 102 before entering thedistillation tower 104, 204. The feed stream 10 may be expanded in theexpander device 102 after exiting the heat exchanger 100. The expanderdevice 102 helps drop the temperature of the feed stream 10 to a levelsuitable for introduction into the distillation tower 104, 204. Theexpander device 102 may be any suitable device, such as a valve. If theexpander device 102 is a valve, the valve may be any suitable valve thatmay aid in cooling the feed stream 10 before it enters the distillationtower 104, 204. For example, the valve 102 may comprise a Joule-Thompson(J-T) valve.

The system may include a feed separator 103 (FIGS. 3-4). The feed streammay enter the feed separator before entering the distillation tower 104,204. The feed separator may separate a feed stream having a mixed liquidand vapor stream into a liquid stream and a vapor stream. Lines 12 mayextend from the feed separator to the distillation tower 104, 204. Oneof the lines 12 may receive the vapor stream from the feed separator.Another one of the lines 12 may receive the liquid stream from the feedseparator. Each of the lines 12 may extend to the same and/or differentsections (i.e. middle controlled freeze zone, and lower sections) of thedistillation tower 104, 204. The expander device 102 may or may not bedownstream of the feed separator 103. The expander device 102 maycomprise a plurality of expander devices 102 such that each line 12 hasan expander device 102.

The system may include a dehydration unit 261 (FIGS. 1-4). The feedstream 10 may enter the dehydration unit 261 before entering thedistillation tower 104, 204. The feed stream 10 enters the dehydrationunit 261 before entering the heat exchanger 100 and/or the expanderdevice 102. The dehydration unit 261 removes water from the feed stream10 to prevent water from later presenting a problem in the heatexchanger 100, expander device 102, feed separator 103, or distillationtower 104, 204. The water can present a problem by forming a separatewater phase (i.e., ice and/or hydrate) that plugs lines, equipment ornegatively affects the distillation process. The dehydration unit 261dehydrates the feed stream to a dew point sufficiently low to ensure aseparate water phase does not form at any point downstream during therest of the process. The dehydration unit may be any suitabledehydration mechanism, such as a molecular sieve or a glycol dehydrationunit.

The system may include a filtering unit (not shown). The feed stream 10may enter the filtering unit before entering the distillation tower 104,204. The filtering unit may remove undesirable contaminants from thefeed stream before the feed stream enters the distillation tower 104,204. Depending on what contaminants are to be removed, the filteringunit may be before or after the dehydration unit 261 and/or before orafter the heat exchanger 100.

The system may include lines 12. Each of the lines may be referred to asan inlet channel 12. The feed stream is introduced into the distillationtower 104, 204 through one of the lines 12. One or more lines 12 mayextend to the lower section 106 or the middle controlled freeze zonesection 108 of the distillation tower 104, 204 to another of the lines12. For example, the line 12 may extend to the lower section 106 suchthat the feed stream 10 may enter the lower section 106 of thedistillation tower 104, 204 (FIGS. 1-4). Each line 12 may directly orindirectly extend to the lower section 106 or the middle controlledfreeze zone section 108. Each line 12 may extend to an outer surface ofthe distillation tower 104, 204 before entering the distillation tower.

If the system includes the feed separator 103 (FIGS. 3-4), the line 12may comprise a plurality of lines 12. Each line may be the same line asone of the lines that extends from the feed separator to a specificportion of the distillation tower 104, 204.

Before entering the distillation tower 104, 204, a sample of the feedstream 10 may enter an analyzer (not shown). The sample of the feedstream 10 may be a small sample of the feed stream 10. The feed stream10 may comprise feed from multiple feed sources or feed from a singlefeed source. Each feed source may comprise, for example, a separatereservoir, one or more wellbores within one or more reservoirs, etc. Theanalyzer may determine the percentage of CO₂ in the sample of the feedstream 10 and, therefore, the content of CO₂ in the feed stream 10. Theanalyzer may connect to multiple lines 12 so that the feed stream 10 canbe sent to one or more sections 106, 108 of the distillation tower 104,204 after the sample of the feed stream 10 exits the analyzer. If theanalyze determines that the percentage of CO₂ is greater than about 20%or greater than 20%, the analyzer may direct the feed stream to the line12 extending from the lower section 106. If the analyzer determines thatthe percentage of CO₂ is less than about 20% or less than 20%, theanalyzer may direct the feed stream to the line 12 extending from themiddle controlled freeze zone section 108. The analyzer may be anysuitable analyzer. For example, the analyzer may be a gas chromatographor an IR analyzer. The analyzer may be positioned before the feed stream10 enters the heat exchanger 100. The feed stream 10 entering theanalyzer may be a single phase.

While the feed stream 10 may be introduced into any section of thedistillation tower 104, 204 regardless of the percentage of CO₂ in thefeed stream 10, it is more efficient to introduce the feed stream 10into the section of the distillation tower 104, 204 that will employ thebest use of energy. For this reason, it is preferable to introduce thefeed stream to the lower section 106 when the percentage of CO₂ in thefeed stream is greater than any percentage about 20% or greater than 20%and to the middle controlled freeze zone section 108 when the percentageof CO₂ in the feed stream is any percentage less than about 20% or lessthan 20%.

The feed stream may be directly or indirectly fed to one of the sections106, 108. Thus, for the best use of energy it is best to introduce thefeed stream into the distillation tower 104, 204 at the point in thedistillation process of the distillation tower 104, 204 that matches therelevant percentage or content of CO₂ in the feed stream.

The feed stream 10 may enter a feed separator 103. The feed separator103 separates a feed stream vapor portion from a feed stream liquidportion before the feed stream is introduced into the distillation tower104, 204. The feed stream vapor portion may be fed to a differentsection or portion within a section of the distillation tower 104, 204than the feed stream liquid portion. For example, the feed stream vaporportion may be fed to an upper controlled freeze zone section 39 of themiddle controlled freeze zone section 108 and/or the feed stream liquidportion may be fed to a lower controlled freeze zone section 40 of themiddle controlled freeze zone section 108 or to the lower section 106 ofthe distillation tower.

The lower section 106 is constructed and arranged to separate the feedstream 10 into an enriched contaminant bottom liquid stream (i.e.,liquid stream) and a freezing zone vapor stream (i.e., vapor stream).The lower section 106 separates the feed stream at a temperature andpressure at which no solids form. The liquid stream may comprise agreater quantity of contaminants than of methane. The vapor stream maycomprise a greater quantity of methane than of contaminants. In anycase, the vapor stream is lighter than the liquid stream. As a result,the vapor stream rises from the lower section 106 and the liquid streamfalls to the bottom of the lower section 106.

The lower section 106 may include and/or connect to equipment thatseparates the feed stream. The equipment may comprise any suitableequipment for separating methane from contaminants, such as one or morepacked sections 181, or one or more distillation trays with perforationsdowncomers and weirs (FIGS. 1-4).

The equipment may include components that apply heat to the stream toform the vapor stream and the liquid stream. For example, the equipmentmay comprise a first reboiler 112 that applies heat to the stream. Thefirst reboiler 112 may be located outside of the distillation tower 104,204. The equipment may also comprise a second reboiler 172 that appliesheat to the stream. The second reboiler 172 may be located outside ofthe distillation tower 104, 204. Line 117 may lead from the distillationtower to the second reboiler 172. Line 17 may lead from the secondreboiler 172 to the distillation tower. Additional reboilers, set upsimilarly to the second reboiler described above, may also be used.

The first reboiler 112 may apply heat to the liquid stream that exitsthe lower section 106 through a liquid outlet 160 of the lower section106. The liquid stream may travel from the liquid outlet 160 throughline 28 to reach the first reboiler 112 (FIGS. 1-4). The amount of heatapplied to the liquid stream by the first reboiler 112 can be increasedto separate more methane from contaminants. The more heat applied by thereboiler 112 to the stream, the more methane separated from the liquidcontaminants, though more contaminants will also be vaporized.

The first reboiler 112 may apply heat to the stream within thedistillation tower 104, 204. Specifically, the heat applied by the firstreboiler 112 warms up the lower section 106. This heat travels up thelower section 106 and supplies heat to warm solids entering a melt trayassembly 139 (FIGS. 1-4) of the middle controlled freeze zone section108 so that the solids form a liquid and/or slurry mix.

The second reboiler 172 applies heat to the stream within the lowersection 106. This heat is applied closer to the middle controlled freezezone section 108 than the heat applied by the first reboiler 112. As aresult, the heat applied by the second reboiler 172 reaches the middlecontrolled freeze zone section 108 faster than the heat applied by thefirst reboiler 112. The second reboiler 172 also helps with energyintegration.

The equipment may include one or more chimney assemblies 135 (FIGS.1-4). While falling to the bottom of the lower section 106, the liquidstream may encounter one or more of the chimney assemblies 135.

Each chimney assembly 135 includes a chimney tray 131 that collects theliquid stream within the lower section 106. The liquid stream thatcollects on the chimney tray 131 may be fed to the second reboiler 172.After the liquid stream is heated in the second reboiler 172, the streammay return to the middle controlled freeze zone section 108 to supplyheat to the middle controlled freeze zone section 108 and/or the melttray assembly 139. Unvaporized stream exiting the second reboiler 172may be fed back to the distillation tower 104, 204 below the chimneytray 131. Vapor stream exiting the second reboiler 172 may be routedunder or above the chimney tray 131 when the vapor stream enters thedistillation tower 104, 204.

The chimney tray 131 may include one or more chimneys 137. The chimney137 serves as a channel that the vapor stream in the lower section 106traverses. The vapor stream travels through an opening in the chimneytray 131 at the bottom of the chimney 137 to the top of the chimney 137.The opening is closer to the bottom of the lower section 106 than it isto the bottom of the middle controlled freeze zone section 108. The topis closer to the bottom of the middle controlled freeze zone section 108than it is to the bottom of the lower section 106.

Each chimney 137 has attached to it a chimney cap 133. The chimney cap133 covers a chimney top opening 138 of the chimney 137. The chimney cap133 prevents the liquid stream from entering the chimney 137. The vaporstream exits the chimney assembly 135 via the chimney top opening 138.

After falling to the bottom of the lower section 106, the liquid streamexits the distillation tower 104, 204 through the liquid outlet 160. Theliquid outlet 160 is within the lower section 106 (FIGS. 1-4). Theliquid outlet 160 may be located at the bottom of the lower section 106.

After exiting through the liquid outlet 160, the feed stream may travelvia line 28 to the first reboiler 112. The feed stream may be heated bythe first reboiler 112 and vapor may then re-enter the lower section 106through line 30. Unvaporized liquid may continue out of the distillationprocess via line 24.

The systems may include an expander device 114 (FIGS. 1-4). Afterentering line 24, the heated liquid stream may be expanded in theexpander device 114. The expander device 114 may be any suitable device,such as a valve. The valve 114 may be any suitable valve, such as a J-Tvalve.

The system may include a heat exchanger 116 (FIGS. 1-4). The liquidstream heated by the first reboiler 112 may be cooled or heated by theheat exchanger 116. The heat exchanger 116 may be a direct heatexchanger or an indirect heat exchanger. The heat exchanger 116 maycomprise any suitable heat exchanger. After exiting the heat exchanger116, the liquid stream exits the distillation process via line 26.

The vapor stream in the lower section 106 rises from the lower section106 to the middle controlled freeze zone section 108. The middlecontrolled freeze zone section 108 is constructed and arranged toseparate the feed stream 10 introduced into the middle controlled freezezone section, or into the top of lower section 106, into a solid and avapor stream. The middle controlled freeze zone section 108 forms asolid, which may comprise more of contaminants than of methane. Thevapor stream (i.e., methane-enriched vapor stream) may comprise moremethane than contaminants.

The middle controlled freeze zone section 108 includes a lower section40 and an upper section 39. The lower section 40 is below the uppersection 39. The lower section 40 directly abuts the upper section 39.The lower section 40 is primarily but not exclusively a heating sectionof the middle controlled freeze zone section 108. The upper section 39is primarily but not exclusively a cooling section of the middlecontrolled freeze zone section 108. The temperature and pressure of theupper section 39 are chosen so that the solid can form in the middlecontrolled freeze zone section 108.

The middle controlled freeze zone section 108 may comprise a melt trayassembly 139 that is maintained in the middle controlled freeze zonesection 108 (FIGS. 1-4). The melt tray assembly 139 is within the lowersection 40 of the middle controlled freeze zone section 108. The melttray assembly 139 is not within the upper section 39 of the middlecontrolled freeze zone section 108.

The melt tray assembly 139 is constructed and arranged to melt solidsformed in the middle controlled freeze zone section 108. When the warmvapor stream rises from the lower section 106 to the middle controlledfreeze zone section 108, the vapor stream immediately encounters themelt tray assembly 139 and supplies heat to melt the solids. As shown inFIGS. 1-4 and more particularly in FIGS. 5-6, the melt tray assembly 139may comprise at least one of a melt tray 118, a bubble cap 132, a liquid130, one or more draw-off openings 130 a, one or more return inlets 146,and optionally may include a heat mechanism(s) 134.

The melt tray 118 may collect a liquid and/or slurry mix. The melt tray118 divides at least a portion of the middle controlled freeze zonesection 108 from the lower section 106. The melt tray 118 is at thebottom 45 of the middle controlled freeze zone section 108.

One or more bubble caps 132 may act as a channel for the vapor streamrising from the lower section 106 to the middle controlled freeze zonesection 108. The bubble cap 132 may provide a path for the vapor streamup the riser 140 and then down and around the riser 140 to the melt tray118. The riser 140 is covered by a cap 141. The cap 141 prevents theliquid 130 from travelling into the riser and it also helps preventsolids from travelling into the riser 140. The vapor stream's traversalthrough the bubble cap 132 allows the vapor stream to transfer heat tothe liquid 130 within the melt tray assembly 139.

One or more heat mechanisms 134 may further heat up the liquid 130 tofacilitate melting of the solids into a liquid and/or slurry mix. Theheat mechanism(s) 134 may be located anywhere within the melt trayassembly 139. For example, as shown in FIGS. 1-4, a heat mechanism 134may be located around bubble caps 132. The heat mechanism 134 may be anysuitable mechanism, such as a heat coil. The heat source of the heatmechanism 134 may be any suitable heat source.

The liquid 130 in the melt tray assembly is heated by the vapor stream.The liquid 130 may also be heated by the one or more heat mechanisms134. The liquid 130 helps melt the solids formed in the middlecontrolled freeze zone section 108 into a liquid and/or slurry mix.Specifically, the heat transferred by the vapor stream heats up theliquid, thereby enabling the heat to melt the solids. The liquid 130 isat a level sufficient to melt the solids.

According to an aspect of the disclosure, the liquid 130 may also beheated by drawing off part of the liquid, heating the liquid throughheat exchange with one or more heat sources external to the controlledfreeze zone section 108, and returning the heated liquid to theremainder of the liquid retained by the melt tray assembly 139. As shownin FIG. 5, one or more draw-off openings 130 a may be included to drawoff part of the liquid 130. The draw-off openings 130 a may bepositioned at or near the bottom of the melt tray 118. At least part ofthe liquid retained by the melt tray exits the draw-off openings 130 aand is pumped, using pump 142, to one or more heat exchangers to heat orwarm the liquid 130. The heat exchangers may include heat exchanger 100,which operates to drop the temperature of the feed stream 10 aspreviously described. The drawn-off liquid 143 may alternatively oradditionally pass through one or more additional heat exchangers(depicted and described herein as additional heat exchanger 144) and beheated by a heating fluid 145. The heating fluid may be ethane, propane,or another suitable fluid, and is returned to heating source (not shown)after passing through the additional heat exchanger 144. Once heated bythe heat exchanger 100 and/or the additional heat exchanger 144, theheated liquid 143 a is returned to the melt tray assembly 139 throughthe one or more return inlets 146. The heated liquid 143 a mixes withthe liquid 130 and provides additional heat to melt solids retained inthe liquid 130.

The use of heat exchanger 100 to heat liquid 130 may eliminate thenecessity of heating element 134. Such elimination of the heatingelement 134 may significantly reduce the congestion and complexity ofthe melt tray assembly 139. Also, heating the liquid 130 in the heatexchanger 100, combined with recycling the heated liquid 143 a back tothe melt tray assembly 139, provides a more even heating operation tothe liquid 130 across a wide range of operating conditions. In contrast,the heating element 134 could provide undesirable non-uniform heating,particularly at turn-down conditions where the temperature pinches outwell before the end of the heating element.

The heat exchanger 100 and/or the additional heat exchanger 144 may beadvantageously used with applications in which low CO₂ feed gas (i.e.,feed gas having a composition of less than 30 mol % CO₂, or less than 25mol % CO₂, or less than 22 mol % CO₂, or less than 20 mol % CO₂) isintroduced above the melt tray assembly 139, as is described, forexample, in United States patent application titled “Hybrid Tray forIntroducing a Low CO₂ Feed Stream into a Distillation Tower,” beingcommonly owned and filed on an even date herewith, the disclosure ofwhich is incorporated herein by reference in its entirety. In such asituation, the heat exchangers may provide beneficial heat integrationbetween the relatively warmer feed gas (about −60° F., which is thelowest achievable temperature using a conventional propane-basedrefrigeration system plus Joule-Thompson cooling via valve 102) and therelatively colder melt tray liquid (about −75° F. to −80° F.). Thiswould provide a means to cool the feed gas while maintaining its low CO₂content. The heated liquid 143 a provides a portion of the heat requiredto melt solids in the melt tray assembly 139, while the additionalcooling of the feed gas reduces the load on the condenser 122 andassociated equipment while the additional heating of the melt trayliquid reduces the heat input needed from other sources—such as theheating element 134.

According to aspects of the disclosure, the pumping rate of pump 142 mayadvantageously be established based on design parameters of thedistillation tower 104, 204, such as tower size, sizes and locations ofdraw-off openings 130 a and return inlets 146, and the like. In anaspect there should be no need to throttle or control the pumping rateduring operation. Additionally, heat input to the additional heatexchanger 144, if present, should be controlled on the side of theheating fluid 145. In a preferred aspect the heat input may be adjustedto maintain a target temperature for the heated liquid 143 a, drawn-offliquid, the liquid 130, or another location.

The duty for heat exchanger 100 should be maximized to provide the mostefficient operation. As a precaution, a feed gas bypass line 147 and abypass valve 148 may be used to permit the feed gas 10 to bypass theheat exchanger 100, thereby increasing the temperature of the feed gas.This option may be used if feed gas risers, which introduce feed gasabove the liquid level of liquid 130, experience fouling from solid CO₂in a low CO₂ environment.

The middle controlled freeze zone section 108 may also comprise a sprayassembly 129. The spray assembly 129 cools the vapor stream that risesfrom the lower section 40. The spray assembly 129 sprays liquid, whichis cooler than the vapor stream, on the vapor stream to cool the vaporstream. The spray assembly 129 is within the upper section 39. The sprayassembly 129 is not within the lower section 40. The spray assembly 129is above the melt tray assembly 139. In other words, the melt trayassembly 139 is below the spray assembly 129.

The spray assembly 129 includes one or more spray nozzles 120 (FIGS.1-4). Each spray nozzle 120 sprays liquid on the vapor stream. The sprayassembly 129 may also include a spray pump 128 (FIGS. 1-4) that pumpsthe liquid. Instead of a spray pump 128, gravity may induce flow in theliquid.

The liquid sprayed by the spray assembly 129 contacts the vapor streamat a temperature and pressure at which solids form. Solids, containingmainly contaminants, form when the sprayed liquid contacts the vaporstream. The solids fall toward the melt tray assembly 139.

The temperature in the middle controlled freeze zone section 108 coolsdown as the vapor stream travels from the bottom of the middlecontrolled freeze zone section 108 to the top of the middle controlledfreeze zone section 108. The methane in the vapor stream rises from themiddle controlled freeze zone section 108 to the upper section 110. Somecontaminants may remain in the methane and also rise. The contaminantsin the vapor stream tend to condense or solidify with the coldertemperatures and fall to the bottom of the middle controlled freeze zonesection 108.

The solids form the liquid and/or slurry mix when in the liquid 130. Theliquid and/or slurry mix flows from the middle controlled freeze zonesection 108 to the lower distillation section 106. At least part of theliquid and/or slurry mix flows from the bottom of the middle controlledfreeze zone section 108 to the top of the lower section 106 via a line22 (FIGS. 1-4). The line 22 may be an exterior line. The line 22 mayextend from the distillation tower 104, 204. The line 22 may extend fromthe middle controlled freeze zone section 108. The line may extend tothe lower section 106. The line 22 may extend from an outer surface ofthe distillation tower 104, 204.

As shown in FIGS. 1-2, the vapor stream that rises in the middlecontrolled freeze zone section 108 and does not form solids or otherwisefall to the bottom of the middle controlled freeze zone section 108,rises to the upper section 110. The upper section 110 operates at atemperature and pressure and contaminant concentration at which no solidforms. The upper section 110 is constructed and arranged to cool thevapor stream to separate the methane from the contaminants. Reflux inthe upper section 110 cools the vapor stream. The reflux is introducedinto the upper section 110 via line 18. Line 18 may extend to the uppersection 110. Line 18 may extend from an outer surface of thedistillation tower 104, 204.

After contacting the reflux in the upper section 110, the feed streamforms a vapor stream and a liquid stream. The vapor stream mainlycomprises methane. The liquid stream comprises relatively morecontaminants. The vapor stream rises in the upper section 110 and theliquid falls to a bottom of the upper section 110.

To facilitate separation of the methane from the contaminants when thestream contacts the reflux, the upper section 110 may include one ormore mass transfer devices 176. Each mass transfer device 176 helpsseparate the methane from the contaminants. Each mass transfer device176 may comprise any suitable separation device, such as a tray withperforations, or a section of random or structured packing to facilitatecontact of the vapor and liquid phases.

After rising, the vapor stream may exit the distillation tower 104, 204through line 14. The line 14 may emanate from an upper part of the uppersection 110. The line 14 may extend from an outer surface of the uppersection 110. From line 14, the vapor stream may enter a condenser 122.The condenser 122 cools the vapor stream to form a cooled stream. Thecondenser 122 at least partially condenses the stream. After exiting thecondenser 122, the cooled stream may enter a separator 124. Theseparator 124 separates the vapor stream into liquid and vapor streams.The separator may be any suitable separator that can separate a streaminto liquid and vapor streams, such as a reflux drum. Once separated,the vapor stream may exit the separator 124 as sales product. The salesproduct may travel through line 16 for subsequent sale to a pipelineand/or condensation to be liquefied natural gas. Once separated, theliquid stream may return to the upper section 110 through line 18 as thereflux. The reflux may travel to the upper section 110 via any suitablemechanism, such as a reflux pump 150 (FIGS. 1 and 3) or gravity (FIGS. 2and 4).

The liquid stream (i.e., freezing zone liquid stream) that falls to thebottom of the upper section 110 collects at the bottom of the uppersection 110. The liquid may collect on tray 183 (FIGS. 1 and 3) or atthe bottommost portion of the upper section 110 (FIGS. 2 and 4). Thecollected liquid may exit the distillation tower 104, 204 through line20 (FIGS. 1 and 3) or outlet 260 (FIGS. 2 and 4). The line 20 mayemanate from the upper section 110. The line 20 may emanate from abottom end of the upper section 110. The line 20 may extend from anouter surface of the upper section 110.

The line 20 and/or outlet 260 connect to a line 41. The line 41 leads tothe spray assembly 129 in the middle controlled freeze zone section 108.The line 41 emanates from the holding vessel 126. The line 41 may extendto an outer surface of the middle controlled freeze zone section 108.

The line 20 and/or outlet 260 may directly or indirectly (FIGS. 1-4)connect to the line 41. When the line 20 and/or outlet 260 directlyconnect to the line 41, the liquid spray may be pumped to the spraynozzle(s) 120 via any suitable mechanism, such as the spray pump 128 orgravity. When the line 20 and/or outlet 260 indirectly connect to theline 41, the lines 20, 41 and/or outlet 260 and line 41 may directlyconnect to a holding vessel 126 (FIGS. 1 and 3). The holding vessel 126may house at least some of the liquid spray before it is sprayed by thenozzle(s). The liquid spray may be pumped from the holding vessel 126 tothe spray nozzle(s) 120 via any suitable mechanism, such as the spraypump 128 (FIGS. 1-2) or gravity. The holding vessel 126 may be neededwhen there is not a sufficient amount of liquid stream at the bottom ofthe upper section 110 to feed the spray nozzles 120.

FIG. 7 is a flowchart depicting a method 700 of cryogenically separatingcontaminants from a feed stream in a distillation tower according todisclosed aspects. At block 702 the feed stream is directed into thedistillation tower. At block 704 vapor is permitted to rise upwardlyfrom a distillation section of the distillation tower. At block 706 asolid is formed in a controlled freeze zone section of the distillationtower. The controlled freeze zone section is situated above thedistillation section. The solid comprises contaminants in the feedstream. At block 708 the vapor from the distillation section into isdirected into liquid retained by a melt tray assembly using at least onevapor stream riser. At block 710 the solid is melted using the liquidretained by the melt tray assembly. At block 712 a portion of the liquidretained by the melt tray assembly is permitted to exit the controlledfreeze zone section. At block 714 the portion of the liquid is heatedthrough indirect heat exchange with a heating fluid in a heat exchanger.At block 716 the portion of the liquid is returned to the melt trayassembly after the liquid has been heated in the heat exchanger.

It is important to note that the steps depicted in FIG. 7 are providedfor illustrative purposes only and a particular step may not be requiredto perform the inventive methodology. The claims, and only the claims,define the inventive system and methodology.

Disclosed aspects may be used in hydrocarbon management activities. Asused herein, “hydrocarbon management” or “managing hydrocarbons”includes hydrocarbon extraction, hydrocarbon production, hydrocarbonexploration, identifying potential hydrocarbon resources, identifyingwell locations, determining well injection and/or extraction rates,identifying reservoir connectivity, acquiring, disposing of and/orabandoning hydrocarbon resources, reviewing prior hydrocarbon managementdecisions, and any other hydrocarbon-related acts or activities. Theterm “hydrocarbon management” is also used for the injection or storageof hydrocarbons or CO₂, for example the sequestration of CO₂, such asreservoir evaluation, development planning, and reservoir management.The disclosed methodologies and techniques may be used to producehydrocarbons in a feed stream extracted from, for example, a subsurfaceregion. The feed stream extracted may be processed in the distillationtower 104, 204 and separated into hydrocarbons and contaminants. Theseparated hydrocarbons exit the middle controlled freeze zone section108 or the upper section 110 of the distillation tower. Some or all ofthe hydrocarbons that exit are produced. Hydrocarbon extraction may beconducted to remove the feed stream from for example, the subsurfaceregion, which may be accomplished by drilling a well using oil welldrilling equipment. The equipment and techniques used to drill a welland/or extract the hydrocarbons are well known by those skilled in therelevant art. Other hydrocarbon extraction activities and, moregenerally, other hydrocarbon management activities, may be performedaccording to known principles.

Aspects of the disclosure may include any combinations of the methodsand systems shown in the following numbered paragraphs. This is not tobe considered a complete listing of all possible aspects, as any numberof variations can be envisioned from the description above.

1. A cryogenic distillation tower for separating a feed stream, thedistillation tower comprising:

a distillation section permitting vapor to rise upwardly therefrom;

one or more lines for directing the feed stream into the distillationtower;

a controlled freeze zone section situated above the distillationsection, the controlled freeze zone constructed and arranged to form asolid from the feed stream, the controlled freeze zone section including

-   -   a spray assembly in an upper section of the controlled freeze        zone, and    -   a melt tray assembly in a lower section of the controlled freeze        zone, wherein the melt tray assembly includes        -   at least one vapor stream riser that directs the vapor from            the distillation section into liquid retained by the melt            tray assembly, and        -   one or more draw-off openings positioned to permit a portion            of the liquid retained by the melt tray assembly to exit the            controlled freeze zone section;

a heat exchanger arranged to heat the portion of the liquid throughindirect heat exchange with a heating fluid; and

one or more return inlets that return the portion of the liquid to themelt tray assembly after the portion of the liquid has been heated inthe heat exchanger.

2. The cryogenic distillation tower of claim 1, wherein the heatingfluid is the feed stream prior to the feed stream being directed intothe distillation tower.3. The cryogenic distillation tower of claim 1 or claim 2, wherein theheat exchanger is a first heat exchanger and the heating fluid is afirst heating fluid, and further comprising a second heat exchangerarranged to heat the portion of the liquid through indirect heatexchange with a second heating fluid.4. The cryogenic distillation tower of paragraph 3, wherein the secondheating fluid comprises one or more of ethane and propane.5. The cryogenic distillation tower of any one of paragraphs 1-4,further comprising a pump for pumping the portion of the liquid exitingthe one or more draw-off openings.6. The cryogenic distillation tower of any one of paragraphs 1-5,further comprising a bypass line configured to selectively permit thefeed stream to bypass the heat exchanger.7. The cryogenic distillation tower of any one of paragraphs 1-6,wherein the one or more return inlets comprise two or more return inletsthat are evenly distributed about a perimeter of the cryogenicdistillation tower.8. The cryogenic distillation tower of any one of paragraphs 1-7,wherein the melt tray assembly includes a heating element to heat theliquid retained in the melt tray assembly.9. The cryogenic distillation tower of any one of paragraphs 1-8,wherein the feed gas comprises less than 30 mol % carbon dioxide.10. The cryogenic distillation tower of any one of paragraphs 1-9,wherein the solid comprises carbon dioxide.11. A method of cryogenically separating contaminants from a feed streamin a distillation tower, the method comprising:

directing the feed stream into the distillation tower;

permitting vapor to rise upwardly from a distillation section of thedistillation tower;

forming a solid in a controlled freeze zone section of the distillationtower, the controlled freeze zone section being situated above thedistillation section, wherein the solid comprises contaminants in thefeed stream;

directing the vapor from the distillation section into liquid retainedby a melt tray assembly using at least one vapor stream riser;

melting the solid using the liquid retained by the melt tray assembly;

permitting a portion of the liquid retained by the melt tray assembly toexit the controlled freeze zone section;

heating the portion of the liquid through indirect heat exchange with aheating fluid in a heat exchanger; and

returning the portion of the liquid to the melt tray assembly after theliquid has been heated in the heat exchanger.

12. The cryogenic distillation tower of paragraph 11, wherein theheating fluid is the feed stream prior to the feed stream being directedinto the distillation tower.13. The method of paragraph 11 or paragraph 12, wherein the heatexchanger is a first heat exchanger and the heating fluid is a firstheating fluid, and further comprising:

heating the portion of the liquid through indirect heat exchange with asecond heating fluid in a second heat exchanger.

14. The method of paragraph 13, wherein the second heating fluidcomprises one or more of ethane and propane.15. The method of any one of paragraphs 11-14, further comprising:

pumping the portion of the liquid exiting the one or more draw-offopenings.

16. The method of any one of paragraphs 11-15, further comprising:

selectively permitting the feed stream to bypass the heat exchanger.

17. The method of any one of paragraphs 11-16, wherein the one or morereturn inlets comprise two or more return inlets that are evenlydistributed about a perimeter of the tower.18. The method of any one of paragraphs 11-17, further comprising:heating the liquid retained in the melt tray assembly using a heatingelement.19. The method of any one of paragraphs 11-18, wherein the feed gascomprises less than 30 mol % carbon dioxide.20. The method of any one of paragraphs 11-19, wherein the solidcomprises carbon dioxide.

It should be understood that numerous changes, modifications, andalternatives to the preceding disclosure can be made without departingfrom the scope of the disclosure. The preceding description, therefore,is not meant to limit the scope of the disclosure. Rather, the scope ofthe disclosure is to be determined only by the appended claims and theirequivalents. It is also contemplated that structures and features in thepresent examples can be altered, rearranged, substituted, deleted,duplicated, combined, or added to each other.

What is claimed is:
 1. A cryogenic distillation tower for separating afeed stream, the distillation tower comprising: a distillation sectionpermitting vapor to rise upwardly therefrom; one or more lines fordirecting the feed stream into the distillation tower; a controlledfreeze zone section situated above the distillation section, thecontrolled freeze zone constructed and arranged to form a solid from thefeed stream, the controlled freeze zone section including a sprayassembly in an upper section of the controlled freeze zone, and a melttray assembly in a lower section of the controlled freeze zone, whereinthe melt tray assembly includes at least one vapor stream riser thatdirects the vapor from the distillation section into liquid retained bythe melt tray assembly, and one or more draw-off openings positioned topermit a portion of the liquid retained by the melt tray assembly toexit the controlled freeze zone section; a heat exchanger arranged toheat the portion of the liquid through indirect heat exchange with aheating fluid; and one or more return inlets that return the portion ofthe liquid to the melt tray assembly after the portion of the liquid hasbeen heated in the heat exchanger.
 2. The cryogenic distillation towerof claim 1, wherein the heating fluid is the feed stream prior to thefeed stream being directed into the distillation tower.
 3. The cryogenicdistillation tower of claim 1, wherein the heat exchanger is a firstheat exchanger and the heating fluid is a first heating fluid, andfurther comprising a second heat exchanger arranged to heat the portionof the liquid through indirect heat exchange with a second heatingfluid.
 4. The cryogenic distillation tower of claim 3, wherein thesecond heating fluid comprises one or more of ethane and propane.
 5. Thecryogenic distillation tower of claim 1, further comprising a pump forpumping the portion of the liquid exiting the one or more draw-offopenings.
 6. The cryogenic distillation tower of claim 1, furthercomprising a bypass line configured to selectively permit the feedstream to bypass the heat exchanger.
 7. The cryogenic distillation towerof claim 1, wherein the one or more return inlets comprise two or morereturn inlets that are evenly distributed about a perimeter of thecryogenic distillation tower.
 8. The cryogenic distillation tower ofclaim 1, wherein the melt tray assembly includes a heating element toheat the liquid retained in the melt tray assembly.
 9. The cryogenicdistillation tower of claim 1, wherein the feed gas comprises less than30 mol % carbon dioxide.
 10. The cryogenic distillation tower of claim1, wherein the solid comprises carbon dioxide.
 11. A method ofcryogenically separating contaminants from a feed stream in adistillation tower, the method comprising: directing the feed streaminto the distillation tower; permitting vapor to rise upwardly from adistillation section of the distillation tower; forming a solid in acontrolled freeze zone section of the distillation tower, the controlledfreeze zone section being situated above the distillation section,wherein the solid comprises contaminants in the feed stream; directingthe vapor from the distillation section into liquid retained by a melttray assembly using at least one vapor stream riser; melting the solidusing the liquid retained by the melt tray assembly; permitting aportion of the liquid retained by the melt tray assembly to exit thecontrolled freeze zone section; heating the portion of the liquidthrough indirect heat exchange with a heating fluid in a heat exchanger;and returning the portion of the liquid to the melt tray assembly afterthe liquid has been heated in the heat exchanger.
 12. The method ofclaim 11, wherein the heating fluid is the feed stream prior to the feedstream being directed into the distillation tower.
 13. The method ofclaim 11, wherein the heat exchanger is a first heat exchanger and theheating fluid is a first heating fluid, and further comprising: heatingthe portion of the liquid through indirect heat exchange with a secondheating fluid in a second heat exchanger.
 14. The method of claim 13,wherein the second heating fluid comprises one or more of ethane andpropane.
 15. The method of claim 11, further comprising: pumping theportion of the liquid exiting the one or more draw-off openings.
 16. Themethod of claim 11, further comprising: selectively permitting the feedstream to bypass the heat exchanger.
 17. The method of claim 11, whereinthe one or more return inlets comprise two or more return inlets thatare evenly distributed about a perimeter of the tower.
 18. The method ofclaim 11, further comprising: heating the liquid retained in the melttray assembly using a heating element.
 19. The method of claim 11,wherein the feed gas comprises less than 30 mol % carbon dioxide. 20.The method of claim 11, wherein the solid comprises carbon dioxide.